1. Field of the Invention
The present invention relates to layout system consisting of a piezoactuator and a flexible circuit board, with at least one electrical and mechanical connection, produced by an electrically conducting adhesive, between a first connection contact on the flexible circuit board and a second connection contact on the piezoactuator, wherein the connection is led through an opening in the flexible circuit board and wherein the surfaces of the connection contacts that are glued together have an essentially parallel and equally oriented normal vector. Additionally, the present invention provides a method for assembling a piezoactuator and flexible circuit board to provide such a layout.
2. Description of the Related Art
Industrial applications of layouts exist in which an electrical and mechanical connection is produced by a glue connection between a flexible circuit board and a piezoactuator. These are known from the prior art and are used, for example, in so-called vibration level meters.
The schematic construction of such a vibration level meter is generally shown in FIG. 6.
The schematic construction in FIG. 6 is neither true to scale nor complete and should only represent a kind of basic diagram of the functioning of a vibration level meter.
The depicted vibration level meter has a tuning fork 10, which is arranged on a membrane 9. The membrane 9 is part of the tuning fork 10, wherein prongs 10a, 10b of the tuning fork 10 are joined directly to the membrane 9 and move toward and away from each other by bending of the membrane 9.
The membrane 9 is generally made from refined steel and can be set in vibration by a piezoactuator 1. The piezoactuator 1 is joined to the membrane 9 via a ceramic tile 7, which is provided between the piezoactuator 1 and the membrane 9. The ceramic tile 7 is provided to adapt the different thermal coefficients of expansion of the membrane 9 and the piezoactuator 1 to each other.
An excitation of the piezoactuator 1 occurs via an electrical voltage, which is applied by a flexible circuit board 3 to the top side of the piezoactuator 1. A bottom side of the piezoactuator 1 is metallized throughout, so that a voltage applied to the top side produces a vibration of the piezoactuator 1 by virtue of a segmentation of the electrode on the top side and a different polarization of these piezosegments. The flexible circuit board 3 in the present example is configured as a so-called flex line, i.e., a flexible springlike circuit board with conductor tracks arranged on it.
The flexible circuit board 3 is joined to the piezoactuator 1 at its contact points by a conducting adhesive, so that a voltage is introduced into the piezoactuator 1 only at the points joined by the adhesive. The piezoactuator 1 as well as the ceramic tile 7 and the membrane 9 are also glued together.
In order to generate a vibration of the tuning fork 10 with the piezoactuator 1 and at the same time detect a frequency shift or attenuation of the tuning fork 10, the piezoactuator 1 is divided into four segments. The piezoactuator 1 is generally round in top view and divided into four quadrants, which can be individually actuated by applying a voltage. To create a voltage, each time two diagonally opposite quadrants are subjected to a voltage, while the other two quadrants are used to detect the resulting vibration. In this way, a vibration can be excited and detected at the same time.
In most of the customary layouts of the related art with a piezoactuator 1 and a circuit board 3, the circuit board 3 is provided with a first connection contact 11 for the applying of the voltage, which is arranged corresponding to a second connection contact 12 on the piezoactuator 1. For this, the first connection contact 11 on the circuit board 3 is generally arranged on the side of the flexible circuit board 3 facing the piezoactuator 1, so that an electrically conducting connection can be produced between the first connection contact 11 and the second connection contact 12 by applying a conducting glue and pressing the two parts together.
Because of the slight gluing surface and the subsequent pressing together, a quantity of adhesive must be measured out that is very small and very precise, or else there is a chance of shorting circuits to the adjoining quadrants of the piezoactuator 1 due to running of glue or squeezing out during the pressing. An exactly reproducible dispensing of a very small quantity of a highly viscous medium can only be done with difficulty, due to the surface forces and temperature dependence prevailing in the case of small volumes, so that a higher production cost is the result.
Furthermore, it is a problem with this procedure that a wrong orientation of the flexible circuit board 3 at the outset can no longer be corrected, since the conductive glue between the connection contacts 11, 12 will become smeared and again there is a chance of short circuits between the individual quadrants of the piezoactuator 1 or the correspondingly arranged contacts of the flexible circuit board 3. Furthermore, it is a negative that the flexible circuit board 3 and the piezoactuator 1 cannot be arranged one on top of the other, due to the glue connection 5 in between. Another negative is that when the flexible circuit board 3 and the piezoactuator 1 are pressed together the glue connections located between them are pressed flat and thus any excess glue is forced out sideways next to the connection contacts 11, 12. This increases the risk of short circuits.
Because of the purely material connection between flexible circuit board 3 and piezoactuator 1, any later loading of the glue connection must be absorbed by cohesion and adhesion forces. Thus, a mechanical loading of the glue connection is also only extremely slight.
After depositing the adhesive, the flexible circuit board 3 must be positioned plane parallel and exactly with respect to the piezoactuator 1. For the above-presented reasons, a position correction afterwards is not possible.
There are approaches in which the flexible circuit board 3 is provided with a first opening in the area of the connection contacts 11, 12, through which excess glue can escape. In these layouts, however, there is still the drawback that the adhesive is first placed on the piezoactuator 1 and then the flexible circuit board 3 is pressed together with it, so that a subsequent correction of the orientation of the flexible circuit board 3 relative to the piezoactuator 1 is not possible. Furthermore, it was discovered that, due to the low gap height of the adhesive used in the gap between the flexible circuit board 3 and the piezoactuator 1, large capillary forces prevail, by which the adhesive is also sucked out between piezoactuator 1 and flexible circuit board 3 across the connection contacts provided. Such a spreading of the electrically conducting adhesive between the piezoactuator and the flexible circuit board is encouraged by minimal unevenness or a minimal deviation from parallel of flexible circuit board and piezoactuator. As pointed out above, this effect leads to short circuits, resulting in a defective overall level meter layout.
The problem of the present invention is to eliminate the above-described drawbacks and provide an improved layout system, as well as an improved process control, as well as an improved method for assembling and manufacturing the same.
Accordingly, there is a need improvements that overcome the noted concerns above. Further, there is also a need to improve process efficiencies.